1. Field of the Invention
This invention relates to an impregnating carbonizing process and apparatus for impregnating a porous shaped body of carbon with tar pitch or resin and carbonizing the tar pitch or resin to produce a carbon material of a high density.
2. Description of the Prior Art
In recent years, composite materials of carbon fiber and carbon materials (hereinafter referred to as C/C composite materials) have come into wide use as new materials in the aerospace and aircraft work for rocket nozzles, aircraft brakes and so forth. Further, attention is paid also to application of such C/C composite materials as a structural material for a high temperature furnace or a tray material which is used in an inert atmosphere because they have characteristics that they are light in weight and high in strength and have a small heat capacity and that they are high in impact strength.
Also with regard to conventional graphite materials, the quality has been improved in recent years, and the demand is increasing for fine materials which are fine in crystal grain and small in quantity of pores.
However, one of the greatest technical subjects in production of such carbon materials resides in how to attain a high density. Particularly, establishment of a technique of achieving a high density in an industrial scale, that is, establishment of a technique of mass production of carbon materials of a high density, is a serious subject.
As a technique of improving the high density of such materials, conventionally a process is employed wherein a porous shaped body is impregnated with a carbonizable substance such as tar pitch or resin and then the carbonizable substance is carbonized. Normally, a porous shaped body is impregnated in vacuum and then baked under the atmospheric pressure.
As a technique of impregnating a porous shaped body with tar pitch in vacuum and carbonizing the tar pitch under a pressure of high pressure gas, such a technique, for example, as illustrated in FIG. 5 is already known. The object of the technique is a C/C composite material, and an original shaped body consists mainly of carbon fiber. Referring to FIG. 5, a shaped body 1 is inserted into a vacuum vessel 2, and then, the shaped body 1 is impregnated with tar pitch in vacuum at a temperature of 200 C. After such impregnation, the shaped body 1 is inserted into a baking furnace 3 in which it is heated to a temperature of 850 C. under the atmospheric pressure to carbonize the tar pitch. Then, the outer face of the shaped body thus obtained is roughened, and then, it is inserted into an airtight can 4 together with tar pitch and impregnated with the tar pitch in vacuum again, whereafter the can 4 is sealed to maintain the inside of the can 4 in a vacuum condition. Subsequently, the thus sealed can 4 is inserted into a high temperature, high pressure furnace 5 in which a pressure of argon gas is applied to the can 4 to heat and pressurize the shaped body 1. Thus, the shaped body 1 is carbonized finally in the conditions of a temperature of 650 C. and a pressure of 10,000 psi (about 700 kg/cm.sup.2). After such carbonization, the can 4 is removed from the furnace 5, and the shaped body 1 is inserted into a high temperature furnace 6 and heated to a temperature of 2,700 C. to graphitize the shaped body 1.
When tar pitch is heated and carbonized in an atmosphere of inert gas such as argon gas under a high pressure, carbon produced at the heating carbonizing step may possibly stick to an energizable member such as a heater to cause a damage to insulation or a short-circuiting accident of the energizable member. In order to prevent such possible trouble, the method which employs such a can 4 for enclosing tar pitch therein as described above or another method which employs a specimen case is adopted. The latter method is disclosed, for example, in Japanese Patent Laid-Open No. 62-84291 and Japanese Utility Model Laid-Open No. 63-57500. As an example wherein a specimen case is employed, an apparatus which is disclosed in Japanese Laid-Open No. 62-84291 is shown in FIG. 6.
Referring to FIG. 6, a high pressure vessel 101 has an upper lid 102 and a lower lid 103 fitted in upper and lower openings thereof. The fitted portions of the high pressure vessel 101 with the upper and lower lids 102 and 103 are held in an airtight condition by a pair of seal members 104 and 104', respectively, and a high pressure chamber 105 is defined in the high pressure vessel 101. A pressure of gas acting upon the lids 102 and 103 is supported by a press frame (not shown), and a pair of heating members 106 and 106' and a heat insulating layer 108 are disposed in the inside of the high pressure vessel 101. The heating members 106 and 106' are each composed of an electric heating resistor wire for heating a work 112 to be processed and have a tubular holder 107. The heat insulating layer 108 is provided to restrain heat from being transmitted from the heating members 106 and 106' to the high pressure vessel 101 and the upper and lower lids 102 and 103.
An airtight chamber 115 is formed in a processing chamber 109 on the inner sides of the heating members 106 and 106' and partitioned by an impermeable partition wall 113.
In the case of the apparatus shown in FIG. 6, the airtight chamber 115 is defined by a tube of an inverted cup shape connected uprightly to the lower lid 103 in an airtight relationship by means of a seal member 114.
Generally, the tubular partition wall 113 is preferably made of a metal material such as stainless steel, inconel, molybdenum or tungsten in order to assure the impermeability to gas. However, depending upon a temperature requirement, it is also possible to employ an inorganic material such as impermeable graphite.
The work 112 to be processed is removably inserted into the airtight chamber 115 of the tubular partition wall 113 by way of a furnace floor 111. Further, the partition wall 113 is provided with a check valve 116 which establishes communication between the inside and the outside of the airtight chamber 115 to permit gas to flow from the outside into the inside of the airtight chamber 115 but prevent gas to flow from the inside to the outside of the airtight chamber 115.
In order to assure a valve function of the check valve 116, a seal member such as an O-ring is sometimes used for the valve section. From the point of view of heat resistance of a spring of the check valve 116, the check valve 116 is preferably disposed at a lower location of the airtight chamber 115 at which the temperature is comparatively low.
According to circumstances, the check valve 116 may be provided in a duct line system which is provided in the inside of the lower lid 103 constituting part of the partition wall for establishing communication between the inside and the outside of the airtight chamber 115.
In the apparatus shown in FIG. 6, duct lines 117, 118 and 119 for communicating the airtight chamber 115 to the outside of the high pressure vessel are formed in the lower lid 103, and an opening and closing valve 120 is provided in the duct line 118.
A further duct line 124 is formed in the lower lid 103 and communicates with the processing chamber 105, and the opening and closing valve 120 is moved to an open position in response to an electric signal from a pressure difference detector 125 which is connected to the duct line 124 and the duct line 119 in the lower lid 103.
Subsequently, a processing method with the apparatus shown in FIG. 6 and functions of the individual members for such processings will be described.
The gas in the inside of the processing chamber 105 of the high pressure vessel 101 is discharged, for example, by way of a duct line 110 formed in the upper lid 102 by means of a vacuum pump (not shown), and after then, inert gas such as argon gas is introduced into the processing chamber 105 similarly by way of the duct line 110.
In this instance, while the outside of the airtight chamber 115 can be put into a vacuum condition by such discharging of the internal gas, the inside of the airtight chamber 115 cannot be put into a vacuum condition due to the presence of the check valve 116. Therefore, in order to discharge the gas from the inside of the airtight chamber 115 until a vacuum condition is reached, the duct lines 117 and 118 in the lower lid 103 are used.
Also in the inert gas introducing operation, it is advantageous to introduce gas by way of the duct line 110 in the upper lid 102 while the duct lines 117 and 118 in the lower lid 103 are utilized to discharge the internal gas in order to accomplish replacement of gas in the airtight chamber 115 perfectly.
After water or oxygen which is bad for materials of the components of the apparatus or the work 112 to be worked is removed by such air discharging and gas introducing operations, argon gas is sent into the inside of the processing chamber 105 to a predetermined pressure by way of the duct line 110.
After the pressure medium gas is filled fully into the processing chamber 105, power is supplied to the heating chambers 106 and 106' to heat the work 112. In this instance, however, the rise in pressure when the temperature rises is greater on the inside of the airtight chamber 115 than on the outside of the airtight chamber 115. Accordingly, an excessive amount of the internal pressure may be discharged outside the high pressure vessel 101 by opening the opening and closing valve 120.
The opening and closing valve 120 is opened in response to an electric signal which is delivered from the pressure difference detector 125 when the difference between the external pressure and the internal pressure of the airtight chamber 115 which is detected by the pressure difference detector 125 reaches a predetermined value.
On the other hand, an improved technique of an HIP (hot isostatic pressing) equipment is disclosed in Japanese Patent Publication No. 58-46524 though not used for impregnation nor carbonization of a carbon material.
The prior art is intended for application to a hot isostatic pressing method for shaping and sintering powder, a method for processing a material for a sintered tool at a high temperature under a high pressure or a high pressure bonding method for bonding a turbine blade to a turbine body. The improved HIP equipment is constructed such that a heat insulating layer, a heater, a work to be processed and a lower lid may be removed in an integral relationship from a high pressure vessel, and a pre-heating operation can be performed outside the HIP equipment without occupying the expensive high pressure vessel. In particular, with the improved HIP equipment, in order to reduce the cycle time of the HIP processing, a work to be processed is placed in advance on the lower lid outside the HIP vessel, and the heater and the heat insulating layer are set in position around the work. In this condition, the heater is energized to pre-heat the same before the work is inserted into the high pressure vessel of the HIP equipment, and after such pre-heating, the work, lid, heater and heat insulating layer are set in position in an integral relationship into the high pressure vessel of the HIP equipment. Consequently, the time required for raising the temperature of the work in the high pressure vessel of the HIP equipment to a predetermined level can be reduced.
The prior art equipments described above, however, have the following drawbacks. In particular, with the arrangement shown in FIG. 5 wherein the shaped body 1 is enclosed in the can 4, the can 4 is contracted and deformed to disable re-use thereof because a pressure of up to 700 kgf/cm.sup.2 is finally applied to the shaped body 1 from the outside of the can 4. Therefore, it is necessary to produce a new can 4 each time the processing described above is to be performed. Accordingly, the cost of consumable goods is increased due to production of such can 4. Further, the expense for sealing operation is also required. Thus, the prior art requires a high processing cost.
Besides, in the process wherein tar pitch is carbonized, gas such as hydrocarbon or hydrogen is generated. Then, if the pressure within the can is increased by the gas thus generated and finally exceeds a pressure of argon gas outside the can, the can may be swollen and broken. In order to prevent this, it is necessary to cause the hydrocarbon in the can to be decomposed rapidly into carbon and hydrogen and raise the temperature while waiting for the hydrogen to be diffused into a wall of the can and pass through the same to the outside of the can. Accordingly, there is a drawback that a long period of time is required for a required temperature rise.
Meanwhile, with the prior art disclosed in Japanese Patent Laid-Open No. 62-84291 and Japanese Utility Model Laid-Open No. 63-57500 wherein a specimen case is used, a series of operations (steps) of melting tar pitch and impregnating a porous shaped body with the tar pitch are performed in the high pressure vessel also as apparently seen from FIG. 6. Then, when the tar pitch is melted, it is heated to a temperature of 200 to 300 C. However, the heat conductivity of tar pitch is as low as a level of that of a resin. Accordingly, a very long period of time is required until the tar pitch is melted, and consequently, the utilization efficiency of the expensive high pressure vessel is very low. In the case of, for example, a specimen having a diameter greater than 20 cm, 10 to 20 hours are required.
To the contrary, the prior art disclosed in Japanese Patent Publication No. 58-46524 is not suitable to impregnate a porous shaped body with tar pitch and carbonize the tar pitch. Thus, in order to perform an impregnating operation at first, it is necessary to heat tar pitch into a molten condition in vacuum. However, since the heater and the work to be processed are disposed in the same spacing, components which will be gasified at a heating and melting step (low boiling point components) will stick to the heater and so forth, which may cause a damage to insulation. Accordingly, the prior art cannot be used actually.